The present invention relates generally to optical transmission, and more particularly to methods and apparatus for mitigating penalties due to intersymbol interference (ISI).
The optical add/drop multiplexer (OADM) is a key enabler for flexible wavelength management in wavelength division multiplexed (WDM) transmission systems. However, high data rate transmission systems (e.g. 40-Gb/s) that include multiple OADMs in series or other devices that provide narrow-band optical filtering can suffer from significant ISI due to the narrow-band optical filtering. ISI is the spreading of the signal in each bit slot to other bit slots. This spreading closes the eye opening in digital transmission systems and leads to degraded system performance.
One prior art solution to mitigate ISI from narrow-band optical filtering is to use a transmission format with a narrower optical bandwidth. However, it is not always feasible or desirable to use narrow-band transmission formats, since such transmission formats can be more sensitive to nonlinearity and noise. Additionally, such solutions are not applicable to systems using conventional (non narrow-band) modulation formats.
Another prior art solution is to use an electronic equalizer. However, the effectiveness of electronic equalization is fundamentally limited because of the loss of optical phase information upon conventional square-law detection. Furthermore, electronic equalization at 40 Gb/s and above normally requires electronics with a bandwidth larger than 80 GHz, and is impractical in many applications.
Another prior art solution is to use an equalizer apparatus to provide optical equalization of a distorted signal after optical filtering. Such an apparatus is discussed in “Simple Multichannel Optical Equalizer Mitigating Intersymbol Interference for 40-Gb/s Nonreturn-To-Zero Signals” by C. R. Doerr, et. al. The proposed optical equalizer is introduced as an additional optical component to compensate for the previously induced ISI penalties, and employs Mach-Zehnder interferometers. Such a solution, however, may not be appropriate for all commercial applications.